Where a travel agent is no cheaper, I would go direct to airline generally, though with recent dealings with booking a SYD-LHR ticket on UA via a travel agent and trying to deal with Qantas for a ticket back to Australia, the travel agent was far better in most areas

Regarding items such as
The OTA owns the booking - any changes MUST be made through them

Once booked, I wanted to make date/time changes to the UA booking and was able to deal directly with UA. If there had been any fare differemce to pay, then would have been up for $50 for airline to take it over, but for simple exchanges, no issue.. Of course if had had to pay the $50, then the airline would have owned the booking and handled all subsequent requests anyway

Some OTA charge cancellation and change fees over that of the airline - That is a case of due diligence when choosing an agent. If the saving is GBP170 and agency charges say, GBP30, service fee - so what. With cancellations where there is an issue , rather than a choice, then thats where travel insurance comes in anyway.

Some OTA have proven to be incredibly difficult to get a refund from and charge fees to issue such - that is where disputing charge with card company comes in. If a refund is due and not processed, the card company will deal with it - ( ok this does assume payment by credit/debit card )

There may be a delay in getting the OTA to respond to any last minute change by the airline - If it is a last minute issue by airline, the airline will , ime, address it anyway

Changes may be communicated to the OTA who fail to onwardly notify you - could be an issue depending on travel agency - for GBP170, I would be happy to make occasional checks that all is ok - I do this anyway for bookings directly with airlines, since I have found airlines can have noticeable delays in notification